Polymer-coated packaging boards are used for packages and containers indented for liquid foodstuffs and other liquid-based or liquid containing products.
An inner polymer layer of the container or package protects the board from being moistened or wetted by liquid or moist substance contained therein. Depending on the polymers used the coating may also provide oxygen or flavour protection for products contained in the package.
An inner cutting edge (raw edge) of the board is exposed to liquid or moist products contained in a container, such as a disposable drinking cup, or a package, such as a milk or juice carton, so that the liquid has the possibility to get absorbed into the fibrous board base through this edge (raw edge penetration).
In an autoclave process there is also the problem of the exposed outer cutting edge absorbing moisture from the hot steam used for sterilization and also from the cold water used for cooling the autoclave-sterilized package.
To avoid these wetting problems the edge of the board can be skived and double folded around the edge, or the raw edges may be hidden by seaming tape. A more cost efficient way is to prevent liquid absorption by suitable sizing of the fibrous board base.
U.S. Pat. No. 4,859,244 describes paper products sized with alkyl ketene dimer (AKD), fatty acid anhydride such as stearic anhydride, and alum. The paper may be laminated on both sides with polyethylene film. The goal is to achieve resistance to raw edge penetration, and the paper is said to withstand alcoholic liquids, 5% aqueous ethanol being given as an example.
WO 2005/003460 A1 describes a packaging material and an autoclave package made thereof, wherein the raw edge penetration has been prevented by use of hydrophobic size such as AKD or alkenyl succinic acid anhydride (ASA), wet-strength size such as polyamido amine epichlorohydrin (PAAE), and an aluminium or calcium compound. The packaging material is aimed to withstand autoclaving with pressurized steam at an elevated temperature.
WO2009/125068 A1 describes a polymer-coated board intended in particular for drinking cups withstanding strong alcoholic drinks with an alcohol content of 30 wt-%. The problem is raw edge penetration on the cup inside, which is solved by a combination of AKD and PAAE sizes and specific requirements for the density of the fibrous base and the minimum amount of coating polymer.
It is known that different hydrophobic sizes give different repellence for different type of liquids. Hydrophobic sizes are in general effective against water, and some will give extra repellence for certain other liquids or compounds dissolved therein. AKD sizes for instance give repellence against lactic acid and are useful in connection with dairy products, but are ineffective against peroxides used for sterilizing the packages before filling. Rosin sizes instead work well with peroxides and are also suitable for disposable drinking cups, which are tarnished by hot coffee if made from AKD sized paperboard only.
It thus happens that there is no all-around size suitable for all liquids that packaging boards are typically used for or brought into contact with. Traditionally the solution has been so called dual sizing, for instance by combination of acidic rosin sizing and neutral AKD sizing. Such a combination is effective in dairy product packaging, in which the rosin size lets the packaging board withstand peroxide sterilization in the filling machine, and AKD size gives resistance to lactic acid that is present in milk and other dairy products being packaged.
However, dual sizing using rosin size and AKD size in combination is difficult, as rosin sizing requires use of alum and a low pH, whereas AKD sizing needs to be done at a neutral or high pH. This means that the pH must be very carefully controlled during board making and there is a high risk of running problems and precipitation of dirt and hydrophobic components in the board machine wet end.
Furthermore, there is still a general need for a sized packaging board that would have resistance including reduced raw edge penetration in different conditions, so as to be suitable for a variety of purposes, namely for packaging liquid foods such as dairy products, for packaging aggressive non-food liquids such as detergents, for disposable drinking cups for coffee and alcoholic drinks, as well as for autoclaved board packages.